The project is designed to study the effect of sterol substitution on the activity of certain membrane-bound enzymes, the fatty acyl CoA desaturases, in the eucaryotic protozoan, Tetrahymena pyriformis. It is hypothesized that the sterol found in the endoplasmic reticulum of these (and other) cells participates in regulation of the fatty acid biosynthesis process, to help govern the eventual fatty acid composition (and therefore the fluidity) of the membranes of this cell. If such regulation is present, it may also help to explain early events in the perturbation of cell physiology resulting from pathogenic accumulation of sterol, as may be the case in atherosclerosis. Such regulation would also be important in understanding the entire process of membrane biogenesis.